The present invention relates to improved green strength of various elastomers. More specifically, the present invention relates to obtaining improved green strength of various elastomers by adding polydimethylbutadiene, copolymers, terpolymers or tetrapolymers thereof to various elastomers to form various blends.
Science and technology in the elastomer field has improved to such an extent that synthetic elastomers have supplemented or replaced natural rubber to a great extent in the fabrication of tires and other rubber products. Stereospecific polymers and particularly synthetic cis-1,4-polyisoprene have demonstrated physical properties similar to and thus are capable of becoming a complete replacement for natural rubber. However, a major deficiency of rubber elastomers including synthetic cis-1,4-polyisoprene is its lack of sufficient green strength required for satisfactory processing or building properties as in the building of tires. The abatement of this deficiency has long been sought by the art and would greatly facilitate in the replacement of natural rubber which is solely produced in tropical climates.
The term "green strength", while being commonly employed and generally understood by persons skilled in the rubber industry, is nevertheless a difficult property to precisely define. Basically, it is that property of a polymer common in natural rubber, which contributes the proper building conditions where multiple components are employed and which result in little or no release of relative movement of the assembled components subsequent to assembly and prior to initiation of the curing operation. Thus, the problem of low green strength, that is the lack of the requisite mechanical strength for processing and fabricating operations necessarily carried out prior to vulcanization with synthetic polymers or copolymers, is lacking. That is, generally with maximum or "peak" stress which the unvulcanized materials will exhibit during deformation is rather low. Hence, unvulcanized strips or other forms of the elastomer are often distorted during processing or building operations. Although numerous additives and compounds have been utilized in association with various elastomers and particularly synthetic cis-1,4-polyisoprene, substantial improvement in green strength has generally not been accomplished.
Green strength has generally been measured by stress/strain curves of unvulcanized compounds. Usually the performance of a green compound is based upon three points of the stress/strain curve, namely the first peak or inflection of the stress, the ultimate or breaking tensile and the percent of ultimate elongation. Improvements in any one or more of these stress properties indicate improved green strength.
Among the various additive compounds or agents which have been utilized to improve green strength or synthetic rubber elastomers are numerous nitroso compounds as set forth in U.S. Pat. Nos. 2,457,331; 2,477,015; 2,518,576; 2,526,504; 2,540,596; 2,690,780; and 3,093,614. Additionally, various dioxime compounds have been utilized such as those set forth in U.S. Pat. Nos. 2,969,341; 3,037,954; 3,160,595; and British Pat. 896,309. Yet another class of additives or compounds are the diesters of 5-norbornene as set forth in U.S. Pat. Nos. 3,817,883 and 3,843,613.
Another prior art patent is U.S. Pat. 3,562,303 to Smith and McFadden, which relates to increased green strength of polyisoprene rubbers or copolymers of isoprene rubbers through a partial cure. That is, the polymer or copolymer actually cross-links and thus is cured by using from 10 percent to 30 percent of the total amount of sulfur required to effect complete vulcanization and from 10 percent to 50 percent of the total amount of accelerator required to effect such vulcanization. Thus, this patent does not relate to a blend of rubber polymers but solely to copolymers wherein any green strength improvement is solely through a partial sulfur cure.